1. Field of the Invention
The present invention relates to the field of preparation of uniform ceramics, and more particularly, to the preparation of low residual porosity.
2. Background Information
Polycrystalline ceramics today are being considered for an increasing variety of applications. Many sophisticated applications require ceramics having very uniform composition and properties throughout the ceramic body. One characteristic which is normally non-uniform in ceramics is the porosity or the spatial distribution of pores. In this sense, for high density ceramics (&gt;98% of theoretical maximum density) porosity refers to the presence of closed pores or voids within the ceramic and not to the presence of a connected pattern of open pores as is present in a "porous" body. This distinction is well understood in the ceramics art. The non-uniform porosity which is present in polycrystalline ceramics generally involves the presence of pore clusters in which a large number of pores are disposed within a small volume of the ceramic which is surrounded by material having a substantially smaller number density of pores.
These pore clusters render the ceramic non-uniform in mechanical properties and in the case of potentially transparent ceramics, also renders them optically non-uniform.
U.S. Pat. No. 4,518,546 entitled "Preparation of Yttria-Gadolinia Ceramic Scintillators by Sintering and Gas Hot Isostatic Pressing" by C. D. Greskovich et al. discloses a method for preparing substantially transparent yttria-gadolinia (Y.sub.2 O.sub.3 /Gd.sub.2 O.sub.3) ceramic scintillators which include other oxides such as europium oxide (Eu.sub.2 O.sub.3), neodymium oxide (Nd.sub.2 O.sub.3), ytterbium oxide (Yb.sub.2 O.sub.3), dysprosium oxide (Dy.sub.2 O.sub.3), praseodymium oxide (Pr.sub.2 O.sub.3) and terbium oxide (Tb.sub.2 O.sub.3) as activators for scintillation. This patent is incorporated herein by reference. These ceramic scintillators are optically quite uniform on a macroscopic scale. The optical uniformity of these bodies also indicates that their mechanical properties are also substantially uniform on a macroscopic scale. While these bodies are optically quite uniform, they still have substantial non-uniformities on a microscopic scale as will be discussed subsequently.
In the process of U.S. Pat. No. 4,518,546, a compact of the desired source oxides is cold pressed at pressures of 3,000 psi to 15,000 psi or higher in order to provide a green compact having a relatively high density. This compact is then heated in a vacuum or a reducing atmosphere such as wet hydrogen (dew point about 23.degree. C.), for example, to a sintering temperature between 1,800.degree. C. and 2,100.degree. C. The sintering temperature is maintained for about 1 hour to about 30 hours to cause extensive densification and produce optical transparency.
The resulting ceramic body is transparent with an in-line optical attenuation coefficient of up to 50 cm.sup.-1 at the main emitting wavelength for the scintillator composition. Unfortunately, while the resulting ceramic body is transparent, its optical characteristics are not uniform and it suffers from the presence of clusters of pores in which the clusters are 50 to 300 or 400 microns across or in "diameter". These clusters of pores, which we refer to as "snowballs", are highly dispersive of incident light and result in a non-uniform optical characteristics for the ceramic body as a whole. These snowballs are much more visible in transmitted light than in reflected light. While there are many applications in which the presence of these snowballs does not interfere with the usefulness of the ceramic body, there are other applications in which the presence of snowballs is highly detrimental. One such application is the ceramic scintillator art where the presence of snowballs interferes with uniform extraction of luminescent light generated by incident x-rays or other source energy. Non-uniform collection of the luminescent light adversely affects the resulting image quality of computed tomography body scanners in which these ceramic scintillators are employed for the medical industry. Consequently, these ceramic bodies, when sliced to provide smaller ceramic bodies which are used as scintillators, produce some smaller bodies having acceptable characteristics and other smaller bodies in which snowballs cause rejection as not meeting specifications. Every ceramic plate which we have made using the sinter-only techniques disclosed in U.S. Pat. No. 4,518,546 has had a number of pore clusters in every cubic millimeter of its volume. The number of pore clusters per 1 mm.sup.3 ranges from at least 5 to a large number. The degree to which the pore clusters interfere with light transmission varies with the number, size and position of the pore clusters in the polycrystalline ceramic.
The individual pores of which a cluster of pores or snowball is formed are micron-to-submicron sized voids within grains of the polycrystalline ceramic structure. It is well known in the ceramics art that once a pore is located within the bulk of a grain of the ceramic, that pore is permanently trapped in that location. While it is theoretically possible for diffusion to transport material through the bulk of the crystal grain to fill the void and thereby collapse the pore, it is well known in the ceramics art, that as a practical matter, such transport is not effective for collapsing enough pores to remove a pore cluster. There is no known sintering process that can produce a highly transparent polycrystalline ceramic body having an optical attenuation coefficients in the visible region below about 10 cm.sup.-1 and which is free of pore clusters.
An alternative preparation technique involves sintering the compact at a temperature between about 1,500.degree. C. and 1,700.degree. C. for between 1 and 10 hours and at least until a closed pore stage is reached. Thereafter, the sintered compact is hot isostatic pressed with an inert gas, such as argon gas, for example, at pressures between 1,000 psi and 30,000 psi at temperatures between about 1,500.degree. C. and 1,800.degree. C. for between one-half and 2 hours. These plates are translucent due to a high density of fine (micron to submicrons) pores and have an inline transmission of about 2% in the visible region for a 1 mm thick specimen.
These plates also suffer from discoloration and composition changes at the surfaces of the ceramic body as a result of the processing steps. The color changes can, to some extent, be compensated by further processing as is explained in U.S. Pat. No. 4,518,546 by annealing the ceramic body in air at about 800.degree. C. to 1,200.degree. C. for between 1 and 20 hours. However, the compositional non-uniformities are not corrected by such annealing. Such uncorrected compositional non-uniformities can be a serious flaw which results in nonuniform luminescence. A number of variations on these processes are discussed in the above-identified patent. A processing method in which the ceramic sample does not suffer from discoloration or compositional changes during the process is desirable.
There is a need for ceramics having uniform properties, such as transparent polycrystalline ceramics which are free of pore clusters for use in systems which require uniform, high optical quality, polycrystalline ceramic materials. There is also a need for a method of reliably producing such ceramic bodies.